1. Field of the Invention
The present invention generally relates to integrated circuits. More particularly, the present invention provides a light shield for shielding the floating diffusion of a complementary metal-oxide semiconductor (CMOS) imager.
2. Related Art
Compared to charge coupled device (CCD) imagers, CMOS imagers are less expensive to produce and consume less power. As such, CMOS imagers are increasingly used for a variety of applications including digital cameras and cell phones. However, the signal-to-noise ratio of CMOS imagers must be improved to compete with CCD devices for high resolution applications.
An illustrative pixel sensor cell 10 of a CMOS imager is depicted in FIG. 1. As is known in the art, the pixel sensor cell 10 comprises a photodiode 12 for capturing incident light 14, a floating diffusion FD 16, a transfer gate TG 18 for transferring charge from the photodiode 12 to the floating diffusion 16, a reset gate RG 20 for resetting the floating diffusion 16 to a predetermined potential, and shallow trench isolation (STI) spacers 22 for isolating the components of the pixel sensor cell 10. The pixel sensor cell 10 is formed on a substrate 24. One of the sources of noise is stray light 14′ that is incident on the floating diffusion 16 adjacent to the transfer gate 18. This issue becomes critical, for example, for electronic shutter pixels. In electronic shutter pixels, the charge must sit on the floating diffusion 16 for a relatively long time (e.g., up to a large fraction of a second). If any stray light 14′ hits the floating diffusion 16 during this time, it changes the image that is read out from the pixel sensor cell 10. Therefore, a light shield is desirable over the floating diffusion 16 to block any stray light 14′.